I used the above formula, which is repeated in many text books and reputable engineering websites, to calculate stress in a curved beam. My result was 9.5 MPa, but the FEA result was over 500 MPa (see attached image).

I've checked my calculations and I'm convinced I haven't made a mistake. I really do not know why there is such a large disagreement between my calculations and the FEA result. I have repeated the FEA process several times and in different ways but the disagreement is constant.